Thermoelectric device



Dec. 14, 1943. v

- THERMOELECTRIC DEVICE Filed April 5, 19.41

17; Q1 yf I 7 linnentop: W/u/AM A. 194?.-

attorney.

w. A. RAY ,3 7 l sate.

'- merits is partly on' account and also rsienreenec. 14, 1943- v ass moo i rnnnuonnnormc nnvrcn William A. Ray, Glendale, Calif. Application April 5, IMLSerial-No. 38?,l03' scams; (or. 136-1-5) My present invention relates to thermoelectric devices, andparticularlyato those of the type adapted for the generation of electrical power 'sufdcient to operate asensitlve switching device I or pilotvalve, in distinction to those of the type employed merely for temperature measurement.

Thermoelectric generators of the iormer ype are' commonly employed in connection wlth'the con: trol or gaseous fuel consuming furnaces. the hot Junction end portion of the device usuall being arranged to be heated by the flame. of'apilot burner provided for. the main burner. 'The power produced by the pilot-burner-heated thermoelecf tric device may conveniently be employedior th energization ofan electrically operated pilot valve controlling a fuel-pressure-operated main valve for the furnace, an obvious advantage being that 7 or electrical energy. An additional advantage of burner should accidentallybecome extinguished,

the resultant cessation the thermoelectric device eflects closure of the furnace valve, therebyrendering the system In the prior art, thermoelectric devices 01 the ype indicated have usually been composed of relatively short thermocouple elements, especially ,when a plurality of pairs ofthese elementswere employed. The reason for the use of short eleoi' the high cost of the materials 'or which they are because of the high watts being available from a pilot-burner-heatedr perature differential between the hot and cold Junctions thus being reduced, the thermoelectric generation is correspondingly reduced. In a typical furnace installation, undesirablev heating oi thecold iimctions is generally due mainly to conduction of heat from the hot Junctions, increase or ambient temperature and radiation from the main burner also contributing to a. minor degree as stant regardless elements;

to the heating or the cold Junctions. it is there fore an object of'my invention to provide a then 'moelectric device constructed oi relatively shor' elements,- which elements, are so composed tha1 thethermoelectric generation is relatively conor theheat gradient along the Another object is to provide, in a thermoelectric device oi the type described, thermocouple ele ments, diflerent portions of which are 01' materials having dissimilar thermoelectric powers 01' positions on a thermoelectric scale wherein the E. M". F. of the various material is plotted against temperature. Such a scale is graphically illusgrated on page 307 or Pyrometric Practice, U.'S;

ureau 01' Standards Technologic Paper No. 170,

a all the commercial alloys hereinafter referred. to

I the system is then independent 01 other-sources filed-February 4, 1941, I have of current generation by reproduced in Part in ment were to rise to that of the original hot junction (between the being included in that scale, which is therefore the accompanying drawing. In my copending application Serial No; 377,274,

disclosed a thermocouple adapted to be heated by the flame of .a pilot burner 501- a furnace main burner, wherein one of the thermoelectric elements ,is or such length that it maybe extended toa region outside the furnace; this element is preferably of an alloy such. as Cope1 or constantan, the electrical resistance of which is not excessive. The other element is relatively short, a, wire of good heat conducting material, such as copper, being provided tor the extension of this element to the exterior of th furnace.

sion, as that metal has somewhat similar thermoelectric properties to that of Chromel or nichrome round that the generation of such a thermocouple is reduced after prolonged heating, on account of the rise in temperature at the Junction between the short element and the copper extension. The reason for this reduction isapparent when one. considers that, if the temperature of the Junction between the copper extension and the shortelethe same temperature as Copel and Chromel elements), the efiective hot Junction would then be that between the copper and the elongated Cop l element (as the short Chrome! element, being at the same temperature preferably in the tice,'such an extreme condition is impossible on account 01 the high thermal conductivity of the copper which constantly conducts heat away from its junction with the short element. It is therefore another object of my invention to provide means for so compensating such a thermocouple that its generation is substantially constant.

Other objects and advantages of my invention will be found inthe description, the drawing, and the appended claims.

For complete understanding of the invention, reference may be had to the following detailed description and accompanying drawing, wherein:

Figure 1 is a diagrammatic view: of a thermoelectric device embody my invention;

Figure 2 is a diagrammatic view of a modified tom thermoelectric device also embodying my invention; and

Figure 3 is a graph (referred to above) indicating the thermoelectric power of various alloys and elements.

Referring first to Fig. 1 of the drawing, the numerals ii and i2 generally indicate a pair of thermocouple members; the upper and lower portions it and M, respectively, of member H being or different materials. and the whole portion [5.

of member I! being of a still different material. Each pair of members is joined at its upper end to form a 'hot junction I6, and the series of pairs or members are connected in electricalseries at their lower ends to form cold" junctions II. The thermopile thus formed is connected by copper leads "to a suitable load indicated by a resistance element IS. The members are form of wires or strips or metals capable of withstanding the heat of an ordinary gas flame, the various portions and leads being united by fusion or welding. A typical thermopile structure is disclosed in U. 8. Patent No. 2,236,609, granted April 1, 1941 to W. R..'Ray.

The portion II as Chromel or high positive power (with respect to platinum) as is indicated in the graph 0! Fig. 3. The upper portion I3 0! member II is preferably of an alloy such as Alumel which occupies a negative position on the'thermoelectric power scale, and the lower portion ll of analloy such as Copel or constantanwhichis also negative but to a higher degree than Alumel.

In the prior art, the members H and I! were usually wholly of alloys Chromel, respectively, thehot Junction had been space somewhat heated due to thermal conduction through the members, and the thermoelectric generation was accordinglyv reduced. .With proheated for a short longed heating thej reduction of available power might be quite serious.

junction ii, the

this member is preferably of such as Copel andwith the result that after of time the cold junctions ,also' bec me throughout its length, would have no thermo-f copper. However, in pracportion I3 is cold. The E. M. F. at the particular temperature difference between the hot and cold junctions can thus be determined by measuring on the graph the distance between the Chromel and A lumel lines at that temperature. When by continued heating the lower portions of the elements also become heated, the temperature the intermediate junction l6 between members that, the generation tion is the difference between the hot junction i6 and the cold junction (which is eflectively at the lower ends or members H and i5) is decreased. However,'the generation is not correspondingly reduced as the temperature of the intermediate junction it between the Copel portion l4 and the Alumel portion l3 (being nearer to the source of heat) has obviously been increased to a greator degree than that of the cold junction, with the result that the Copel (which has negative thermoelectric power higher than that of Alumel) becomesefiective to influence the generation.

The reason for the compensating eilect produced by the Copel portion will be best understood by assuming an extreme condition wherein It and i4 is at the same temperature as the'hot junction it. when such is the case, the Alumel portion II has no eilect on the generation as it is at the same temperature throughout its length, r

and the thermoelectric by the distance lines at the particular temperature diflerence between the hot and cold junctions, it being noted power can be determined highest indicated on the graph. However, under the extreme heating conditions assumed, the cold Junction also becomes considerably hotter, the net result being a balance between the gain atthe hot junction and cold junction. It will thus be seen that, in the construction shown in Fig. 1, there is a constant balance oi? the thermoelectric generation regardless of the temperature difference between the hot is preferably of an alloy such nichrome which has a relatively result may bev produced from other parts or the u oi the junctions.

I order on .the scale,,g s .ior example,

portion II of Alumel, and

However, the general and cold junctions when heat is continuously applied to the hot junction. This improvement results from the provision of material for the lower portion ll of member H which has a higher thermoelectric power with respect to that oi member it than that or the material of the upp r portion II; the relative portion is adjacent the hot Junction I. is subjected to the flame.- Obvio the same'general by choosing materials the same relative order; or

member lIoiCopel, portion ll oli'Chromel. arrangement. described above is preferred; parthat the alloys Alumel or oxidization) other-than those shown in the graph .01: Fig. 3 may. the'thermocouple is to be heated to temperatures lower than that produced by a gas flame, or when "cold (instead of heat) is employed for producing the necessary temperature differential, as inconnection with a refrigeration system. It is to be understood that the terms "hot junction" and cold junction are interchangeable under some conditionsand are employed herein to conveniently indicate relative different temperature between the Copel and Chromel of a Copel-Chromel combinathe loss at the lengths or members it and M depending mainly on how much or the thermoelectric scale in by reversing by making be'employed, especially when I Seti'orth beloware examples of conditions under which constant E. M. F. is produced by a single thermocouple oi the thermopile shown in Fig. 1 at progressively varying temperatures of the intermediate and cold junction-it being assumed that the hot junction temperature'remains the same. The E. M. F. of the thermocouple is Y calculated by adding, algebraically, the E. M. F.s generated at each of its junctions at the given temperatures, as indicated in the graph of Fig. 8. In this method of calculation, the relations of the elements at each of the Junctions are set down in sequence around the circuit-conveniently, in this particular-instance, in a counterclockwise direction in order to obtain a result having a plus value. The copper member l8 has not been included because the interposition of an additional element when both ends are at the same temperature does not aflect the voltage of the circuit. It will be understood that, while the exactly-equal results shown by these examples are theoretical, in practice the results are substantially the same over a considerable range of temperature with proper proportioning of the three elements to meet the particular condition under which heat is applied to the hot junction.

Example 1 Millivolts I Junction 16: Chromel-Alumel at500 C..- +210 Junction 16': Alumel-Copel at 20 C +0.3 30 Junction 17: Copel-Chromel at 20 C 1.3'

' +200 Example 2 I Millivolts Function 16: Chromel- A1umel at 500 0.. +210 Function 16': Alumel-Copel at 75 C +2.0 function 17: Copel-Chromel at 45 C I 3.0

I +2o;o

Example 3 I Millivolts unction 16: Chromel-Alumel at 500 C' +21.0

unction 16': Alum'el-Copel at 150 C +4.5 45 unction l7: Copel-Chromel at 80 C +5.5

' 1 +200 Example 4 r I I I Millivolts motion 16: Chromel-Alumel at 500 C'... +210 motion 16?: Alumel-Copel at 220 C +7.0

motion 17: Copel Chromel at 120 C--- 8.0I

. T+20.0 hile the intermediate junction]? is shown'in e drawing as being relatively near. the hot notion l6, it does rature on'account of the relatively low thermal nductivity'of the Alumel portion; preferably. .60 hr the hot-Junctionextremity of this portion .ng subjected to heat .tomparing the results obtainedinxthe above imples with those which would be obtained ier the same conditions 0! temperature dii-f ential ,with a thermocouple constructed -oi' rely Chromel'and n'iumel, wanna the r new-I3}; Y

E. M. F. values: I I summer I 70 I M1llivoltsw 7Junction: Chromel-Alumel at 500 (7..v +214) :1 Junction: Alumel-Chromel at 20 C- -1.0 I

. pensating arrangement pile structure referred not attain a very high tem- I element. JQUDOR .iunction'ifzfi.

'tion 20and1the copper port lonlfl' I resultant decrease inflth'ermoelectricgeneration.

Example 2a 1 Millivolts Hot Junction: Chromel-Alumel at 500 C- I Cold Junction: Alumel-Chromel at C 2.0

The function of the thermopile shown in Fig.

1 is similar to that of the single pair of members discussed above, the advantage of the comeven greater when it is because of-the isolation of the intermediate cold junctions i1. Ina single connecting leads of copper; thecold junction does Junction by the leads. In the patented thermoto above, the members are conveniently about two inches long.

Referring now to various elements ment of Fig.1

Fig. 2 of the drawing, the orresponding to the ele- 2! preferably being of an alloysuch as Alumel, I

ber 22 of Chrome] or nichrome. 'The main 'dif- I portion of a Chromel-or-nichrome thermocouple prolonged. heating, of the hot diate I Junction 26f The relative the members to suit the particular condiof my invention being applied to a thermopilethermocouple having I the temperature at the interme- I ,betweedthe Chromel' per-) risesg withf The. total thermoelectric as being shorter than of elongated members junction .diate junction and the posite ends of that the temperature oisaid cold junction is in-f sai'd hot junction, on

' from that of. the first-mentioned member, said tions to which the th rmocouple ,is subjected, such as the position of the flame with respect to the thermocouple, ambient temperature near the upper end of the thermocouple, and that at the load. As an example, the portion 23 is shown.

the portion 25. g

It is to be understood that modifications other than those mentioned hereinabove may be made without departing from the spirit of my invention and I intend therefore to be limited only by: the scope of the-appended claims.

I claim as my invention:

1. A thermocouple adapted to generate substantially constant voltage and comprising a pair joined at one of their respective ends to form a "hot junction, the opposite ends of the members iorming effectively a cold junction, said members being so short that said cold junction is substantially afiected by the temperature of said hot junction, one of said members being wholly of metal having one thermoelectric power, the otherpf the members being composed of two sections joined longitudinally to form an intermediate junction, the first section between the hot junction and the intermediate being of metal having a thermoelectric power difiering from that of thefirst-mentioned member, the other section between the intermecold' junction being of metal having a thermoelectric power also differing from that of the first-mentioned member and in the same sense as that of said first section but to a greater degree.

-2. A thermocouple adapted togenerate substantially constant voltage and comprising a pair I of elongated members joined at one of their respective ends to iorm a "hot junction the opthe member's forming eflectively a cold junction, said members being so short creased substantially upon prolonged heating of e of said members being wholly of metal having one thermoelectric power, the other of the members being composed of two 45 sections joined longitudinally to form an intermediate junction, the first section between the hot junction and the intermediate junction being of metalhaving a thermoelectric power diiiering first section being. of such'length that said intermediate junction attains .a temperature inter mediate that of said hot and cold junctions upon prolonged heating of said hot junction, the other section between'the intermediate junction and M the cold j'ULJtiOfl being of metalhaving a thermoelectric power also diiiering from that of the first-mentioned member and in the same sense as that of said first section but to a greater deegree. 3. A thermocouple ,adapted ,to generate sub stantially constant voltage and comprising a pair of elongated members joined at one of their respeotive ends to form a hot Junction, the op"- posite ends of the members'iorming eiiectively a. 65 fcold junction," said members being so short that the temperature or said cold junction is increased substantially upon prolonged heating 01' said hot junction,

electric power,

tions' pon prolonge ooo' one of said members being wholly of metal chosen from the group consisting of Chromel and nichrome, the other oi the members being composed of two sections joined longitudinally to-form anintermediate junction, the first section between the hot junction and the intermediate junction being of Alumel, said first section being oi! such length that said intermediate junction attains a temperature intermediate that of said hot and cold junction heating of said hot junction, the' other section between the intermediate junctionand the cold junction being of me i chosen from the group consisting oi Copel and constantan 4. In a thermoelectric device for the generation oi substantially constant voltage: a plural- 20 and of "cold junctions, said members being so short that the temperature of said cold junctions is increased substantially upon prolonged heating of said hot junctions, one member of eachfoi said pairs being wholly 01 metal having one thermo the'other ing composed or two sections joined longitudinally to form an intermediate junction, the first section ,0! each or said other members between the hot junctoin and the intermediate junction sc-being or metal iering from that of the first-mentioned memhaving a thermoelect her, :said iii-st section being of "such length that said intermediate junction attainsfa temperature intermediate that of said not and cold junction, the other section of each oi said other members between the intermediate junction and the cold junction being of metal having a thermoelectric power also differing from that oithe first-- 40 mentioned member and in the same sense I as that of said first section but to a greater degree.

5'. In athermoelectric device torthe generation oi substantially constant; ity oi thermocouples each comprising ,a pair of elongated members and joined in electrical series to term a thermopile having a series 01 "hot" and of .cold" junctions, said members being so short that the temperature or said cold junctions is increased substantially upon prolongedheating of said hot junctions, one'xnei'ribei'v or each'ci said pairs being wholly of memng osen Iromthe group'consisting oi Chi-omel andnichrome, the

other member of eachof two sections joined lonfi tudinfllr to form an in- .termediate junction; the firstsection or each 01 said other members betweenv the hot-junction and the intermediate junction or Alumel; said first section being or such length thatsaidintermediate junction attains a temperature intermo- 7 hot and oold junctionsupon prolonged' heating oi saidhot section '01 each of said the intermediate junction being of metal chosen troni oi Copel and oonstantan.

s upon prolonged member of each pair beric power diid heating of said hot iuncvoltage: a pluralthe other the-cold? junction 

